Signed to a three-year contract with the Kings in July 2002,[1] Frolov made his NHL debut in 2002 for the Kings, scoring his first NHL goal, a game-winner, in his seventh game on October 25 against Mike Richter of the New York Rangers.[2] Frolov finished his rookie season with 14 goals and 31 points. After improving to a team-high 24 goals[3] and 48 points the following season, Frolov returned to the Russian Super League in 2004–05 due to the NHL lockout. He began the season with CSKA Moscow and finished second in team scoring to Nikolay Zherdev, despite completing the campaign with Dynamo Moscow.[4]

Frolov became a restricted free agent before the 2005–06 season and after turning down a three-year deal, he signed a longer, five-year, $14.5 million contract in August 2005.[5] In the first year of his new contract, he scored his first NHL hat-trick on November 13, 2005, in an 8–2 victory over the Columbus Blue Jackets.[6] Later that season, Frolov was named to Russia's 2006 Winter Olympic team in Turin, where he suffered a partially separated shoulder. He missed approximately a month with the Kings when he returned,[7] but still managed a 54-point season in 69 games.

In 2006–07, Frolov continued to improve and recorded a career-high 35 goals and 71 points, second in team point-scoring to Michael Cammalleri.[8] He finished second in team-scoring for the second consecutive year to Anže Kopitar in 2007–08 with 67 points,[9] despite missing 11 games due to a groin injury suffered in November.[10]

An unrestricted free agent after the 2009–10 season, Frolov signed with the New York Rangers on July 27, 2010, on a one-year, $3 million contract. On January 8, 2011, he suffered a torn anterior cruciate ligament (ACL) of the right knee after Anaheim Ducks forward Brad Winchester fell over Frolov's leg. The injury forced him to miss the remainder of the 2010–11 season after he underwent surgery to repair the damage. Frolov registered seven goals and nine assists for 16 points, along with eight penalty minutes, in 43 games during his only season with the Rangers.

On September 15, 2014, Frolov announced his retirement from professional hockey, citing the effect of knee injuries he had sustained since 2011, however after a season out from the game, Frolov returned to professional hockey on July 7, 2015, signing a one-year contract with former club, Torpedo Nizhny Novgorod of the KHL.[13]

After completing his rookie season with the Los Angeles Kings, Frolov debuted at the senior level with Russia at the 2003 World Championships and scored five points as Russia failed to medal. He made his second World Championships appearance in 2007, tallying an international personal best 11 points in nine games. Playing as the host nation, Russia earned bronze defeating Sweden 3–1 in the bronze medal game.

Frolov also competed in the 2004 World Cup, where he managed two assists in four games. He was selected to his first Winter Olympics for the 2006 games in Turin. However, Frolov was injured early in the tournament, suffering a partially separated shoulder in a round-robin game against Kazakhstan.[citation needed] He missed the remainder of Russia's games as they were defeated 3–0 by Finland in the semi-final game.